1. Field of the Invention
The present invention relates to a method and device for detecting an abnormality of a nitrogen oxide purifying catalyst used for reducing and purifying nitrogen oxide contained in exhaust gas using a reducing agent, the method and device employing an NOx sensor for detecting nitrogen oxide concentration.
2. Description of the Related Art
In the past, a technology has been known which detects an abnormality of a catalyst (NOx absorbing catalyst) used for accumulating nitrogen oxide contained in exhaust gas discharged from an internal combustion engine such as an automobile engine, and which employs an NOx sensor for detecting nitrogen oxide concentration (NOx) (for example, see Patent Document 1). In recent years, a technology has also been known which purifies and changes a nitrogen oxide into a harmless component by disposing a nitrogen oxide purifying catalyst, used for reducing and purifying the nitrogen oxide contained in exhaust gas using a reducing agent, in a discharge passage of an internal combustion engine.
[Patent Document 1] JP-A-H11-258194
3. Problems to be Solved by the Invention
However, in an NOx sensor (particularly, an NOx sensor including a plurality of cells each having a solid electrolyte and a pair of electrodes), in order to accurately measure nitrogen oxide concentration on the basis of a sensor signal (i.e., a sensor signal corresponding to a value within a target measurement concentration range), a predetermined time must have elapsed after the internal combustion engine is activated and the NOx sensor begins to generate an output signal. Hereinafter, this time is also referred to as a light-off time. The output signal tends to change during the light-off time. That is, in the NOx sensor, there is an unstable period during which the output signal changes independent of the concentration of the nitrogen oxide once the sensor begins to generate an output signal. For this reason, a problem arises in that an abnormality of the nitrogen oxide purifying catalyst cannot be precisely detected using the NOx sensor immediately after the NOx sensor begins to generate an output signal.